1. Field of the Invention
The present invention relates to flame-retardant epoxy resins, and more particularly to a flame-retardant epoxy resin modified by a reactive phosphorus-containing compound.
2. Background of the Invention
Conventional flame retardant compositions are generally added with flame retardant materials such as halogen-containing compounds or antimony- or vanadium-containing oxides to form high heat resistant compositions. The use of these materials, however, often causes serious environmental pollution problems.
In the application of composite materials, as for example in coating, electrical insulation, construction, building materials, adhesives or laminated products, epoxy resin composite materials are currently frequently used. Because of easy processing, high safety, excellent mechanical and chemical properties, epoxy resins have been widely used in many fields, and especially in the manufacture of laminated plates. Also, because epoxy resins have strong adhesion to reinforcement materials (such as glass-fiber fabrics), no volatility matters, and small shrinkage of the product while hardening, laminated plates produced by such resins have been massively applied to electrical and electronic products or components. However, in order to prevent the products or components from deterioration caused by the elevated temperature during process or use, additives such as, for example, halogen compounds, phosphorus compounds or oxides of antimony, vanadium, etc., are generally added to increase the heat resistance of the epoxy resins.
In the current significant process of the printed circuit board, the laminated plates are required to have excellent electrical properties, mechanical properties, and heat resistant processing properties, in addition there is increased demand for finer patterns on the circuit board. For example, FR4 laminated plates, which are widely used, have a glass transition temperature of about 130° C. after hardening. For the process of the printed circuit board, the temperature is over 200° C. during cutting and drilling, and even reaches over 270° C. during welding. Thus the laminated plates may break or crack during manufacture. In addition, for the laminated plates of printed circuit boards, another important property is flame retardancy. In some applications, such as airplanes, automobiles, and public transportation, the flame retardant property of the printed circuit board is absolutely necessary.
In order to impart a flame retardant property to laminated plates, substances that separate the flame and decrease burning are introduced. For the laminated plates of epoxy resin/glass-fiber systems (or organic fiber), halogen-containing compounds, especially bromine-containing epoxy resins, and hardeners are used in combination with flame retardant material such as antimonous acid anhydride, etc., so that the achievement of flame retardant standards (as strict as the UL94V-0 level) in the laminated plates can be attained. The flame retarding substance in epoxy resin usually contains up to 17% to 21% of bromine and is used in combination with antimonous acid anhydride or other retardants to achieve the standard of UL 94V-0. However, the use of high contents of bromine in epoxy resin or antimonous acid anhydride will adversely affect the health of human body. As well, bromine not only generates erosive free radicals and hydrogen bromide when it burns, aromatic compounds with high bromine contents also produce toxic brominated furans and a brominated dioxine compound during the burning process. Thus, since both human health and the environment are seriously affected, it is necessary to find a novel flame retardant material and a flame retardant property to solve the pollution and environmental problems caused by the current use of laminated products or components containing bromide or bromo-epoxy resin. Especially, it is necessary to actively develop flame retardant epoxy resin materials which will not endanger humans and are environmentally friendly to respond to the massive use of electrical and electric products and components containing FR-4 epoxy glass fiber laminated plates.
Thus, in order to protect human health and decrease environmental pollution, a reactive type of flame retardant material has been developed. The technique using phosphorus-containing compounds instead of bromine-containing compounds (flame retardant materials conventionally used) is widely accepted. The study and application of phosphorus system compounds for use as flame retardant materials that are environmentally friendly are currently extensive; for example, red phosphorus or phosphorus-containing organic compounds (such as triphenyl phosphate, triphenylmethyl phosphate, etc.) are directly added to substitute halide as a flame retardant to improve the burning property of polymer materials or hard resin materials. However, it is necessary to add such compounds in large amounts directly into resins to achieve flame retarding. The high migration due to the smaller molecular weight of the compound will affect the properties of resin substrates making, for example, electrical property, adhesive strength, etc., inferior, resulting in difficulty of application.
It is known that the technique employs a reactive type of phosphorus-containing epoxy resin instead of the bromine-containing or bromo-epoxy resins generally used as flame retardant ingredients, for example in U.S. Pat. No. 5,376,453, which utilizes the composition of phosphates having epoxy group and nitrogen-containing hardener to make laminated plates. It is necessary to add many kinds of phosphate-containing epoxy resin compositions to increase a phosphorus content that will achieve the high standard of UL 94V-0. In U.S. Pat. No. 5,458,978, a composition formed from epoxy phosphates, nitrogen-containing epoxy resin, and metal complex hardener is used to make laminated plates which has the flame retardant property approaching to UL 94V-0 (40 seconds, 50 seconds related to critical value). In U.S. Pat. Nos. 4,973,631 and 5,086,156, a trialkyl oxide having an active hydrogen substituent (such as amino group) is used alone or in combination with other amine hardeners to harden epoxy resins. In these hardening methods of introducing phosphorus to resins, the low content of phosphorus is not able to achieve the flame retardant effect. There is no actual measurement of the flame retardant effect in these two patents.
In the techniques disclosed above it is usually necessary to add many types of phosphorus-containing compounds to meet the flame retardant standard; however, additional additives usually make the processing conditions of products hard to control or the property of their products inferior.
Therefore, the method currently used to increase flame retardant property is to add nitrogen and phosphorus-containing substances instead of the currently used bromine- or antimony-containing substances. The nitrogen-containing substance frequently used is melamine, DICY, or cyanate which contains a triazine ring. The phosphorus-containing compound frequently used is a non-reactive phosphorus-containing compound or a phosphorus-containing compound which has a reactive functional group. Because the heat resistance of the non-reactive phosphorus-containing compound is inferior to the reactive phosphorus-containing compounds and is also more stable for reactive phosphorus-containing compounds binding to other molecules, the current flame retardant materials used are mainly those using a reactive phosphorus-containing compound. The reactive phosphorus-containing compound frequently used is linear. The heat resistance of reactive phosphorus-containing compound is inferior to that of the halogen-containing or non-halogen-containing epoxy resin composition, due to the —O—P—O— bonds on the backbone chain.
The present invention discloses an epoxy resin modified with a side chain having a reactive phosphorus-containing compound, wherein the reactive site which can react with epoxy group in the epoxy resin in the structure is the phosphorus compound on the side chain. The epoxy resin has better heat resistance than that of normal phosphorus-containing compound or epoxy resin, is suitable for flame retardant composition, and can replace the materials containing halogen compounds without affecting the processing property while it achieves environmental protection.